1. Field of the Invention
The present invention relates to a cylindrical alkaline storage battery including a group of spiral electrodes composed of positive and negative electrode plates spirally wound with a separator interposed therebetween, and more particularly to an improvement of an internal structure of the group of spiral electrodes.
2. Description of the Prior Art
In recent years, a cylindrical alkaline storage battery of higher capacity is required for use in various kinds of portable electronic apparatuses and communication equipments. To satisfy the requirements, there have been proposed various methods for impregnating an active material at a higher density in a cell casing of limited volume and for producing the separator as thin as possible. Accordingly, an occupancy ratio of the group of electrodes in the cell casing tends to be increased. This results in an increase of an occurrence ratio of short-circuit in the cell casing due to an increase of pressure force applied to the group of electrodes in the cell casing. To avoid such a problem, it has been proposed in Japanese Patent Laid-open Publication No. 5-234598 to provide a group of spiral electrodes in which a separator is covered with an adhesive tape of alkali resistance at the outside of a positive electrode plate placed at an outermost periphery of the group of spiral electrodes to restrain the occurrence of short-circuit in the cell casing.
To investigate the cause of internal short-circuit in the cell casing, the inventor disassembled storage batteries 30, 40 and 50 respectively shown in FIGS. 3, 4 and 5, where an internal short-circuit occurred therein. As a result, in the storage battery 30 including a group of spiral electrodes B composed of positive and negative electrode plates 31 and 32 spirally wound with a separator 33 interposed therebetween and contained in a cell casing 30a as shown in FIG. 3, an internal short-circuit was found at a point X between a winding end 31a of the positive electrode plate 31 and a portion of the negative electrode plate located at the outside of winding end 31a in a condition wherein the winding end 31a was placed on a maxim diametric line bxe2x80x94b of the group of spiral electrodes.
In the storage battery 40 including a group of spiral electrodes C composed of positive and negative electrode plates 41 and 42 spirally wound with a separator 43 interposed therebetween and contained in a cell casing 40 as shown in FIG. 4, an internal short-circuit was found at a point Y between the positive electrode plate 41 placed on a maximum diametric line cxe2x80x94c of the group of spiral electrodes 40 and the negative electrode plate 42 in a condition where a winding end 41a of positive electrode plate 41 was positioned on a line dxe2x80x94d displaced from the maximum diametric line cxe2x80x94c.
In the storage battery 50 including a group of spiral electrodes D composed of positive and negative electrode plates 51 and 52 spirally wound with a separator 53 interposed therebetween and contained in a cell casing 50a as shown in FIG. 5, an internal short-circuit was found at a point Z between the positive electrode plate 51 placed on a maximum diametric line exe2x80x94e of the group of spiral electrodes D and the negative electrode plate in a condition where a winding end 51a of positive electrode plate 51 was positioned on a line fxe2x80x94f displaced from the maximum diametric line exe2x80x94e.
From the foregoing facts, it has been found that even if the separator was covered with an adhesive tape of alkali resistance adhered thereto at the outside of the positive electrode plate placed at an outermost periphery of the group of spiral electrodes as proposed in Japanese Patent Laid-open Publication 5-234598, the adhesive tape would be ineffective to restrain an occurrence of internal short-circuit in the storage battery. It has been also found that the adhesive tape disturbs an internal reaction in the storage battery as it is not hydrophilic, resulting in a decrease of the battery capacity.
It is, therefore, a primary object of the present invention to provide a cylindrical alkaline storage battery wherein the separator is reinforced by an additional separator of the same kind of material as that of the separator adhered thereto to avoid an occurrence of the internal short-circuit discussed above without causing any decrease of the battery capacity.
According to the present invention, the object is accomplished by providing a cylindrical alkaline storage battery including a group of spiral electrodes composed of positive and negative electrode plates spirally wound with a separator interposed therebetween, wherein a winding end of the positive electrode plate is positioned in a maximum diameter portion of the group of spiral electrodes, and wherein the separator is reinforced by an additional separator of the same kind of its material adhered thereto at an outside of the winding end of the positive electrode plate.
In the case that the winding end of the positive electrode plate is positioned in the maximum diameter portion of the group of spiral electrodes in which a pressure force applied from the cell casing increases more than that in the other portion, an internal short-circuit will occur between the winding end of the positive electrode plate and the negative electrode plate opposed to each other through the separator. For this reason, the reinforcement of the separator at the outside of the winding end of the positive electrode plate is effective to restrain the occurrence of internal short-circuit in the storage battery As the additional separator of the same kind of material as that of the separator is hydrophilic, the additional separator is useful to effectively restrain the occurrence of internal short-circuit without casing any decrease of the battery capacity.
In a practical embodiment, it is preferable that the positive electrode is made of an electrode substrate plate in the form of a porous metal plate such as a foamed nickel plate having a three dimensional mesh-structure impregnated with an active material and rolled under pressure. In use of the positive electrode plate, the additional separator is more useful to prevent an occurrence of internal short-circuit caused by bur fragments of the positive electrode plate. Particularly, in the case that the electrode core plate impregnated with the active material was cut into a predetermined length after rolled under pressure, numberless bur fragments will occur at the cut edge of the positive electrode plate. In such a case, the additional separator becomes more useful to prevent an occurrence of short-circuit in the storage battery.
In case that the winding end of the positive electrode plate placed at the outermost periphery of the group of spiral electrodes is not secured in place by means of a separator or an adhesive tape, the winding end of the positive electrode plate is wound off due to spring-back phenomenon thereof. This causes a short-circuit in the storage battery. Accordingly, in the case that the winding end of the positive electrode plate is secured in place by means of the additional separator adhered thereto, the additional separator becomes useful to prevent an occurrence of short-circuit as the winding end of the positive electrode plate.